The present invention relates to optical reader-scanner systems and, in particular, for improved means for generating optical scan patterns.
Optical reader-scanner systems have achieved applications at automated supermarket check-out counters. A reader-scanner system operates as a data input system for electronic cash register systems and is used to read UPC (universal product code) symbols on the items.
The UPC symbol system was developed by the Universal Grocery Product Code Council, Inc., and is a bar code system which provides for binary coding of ten decimal digits. The first five of these digits identify the producer of the item, and the last five identify the particular item of his product line. The actual symbol of the group is more than sixty parallel light and dark bars. Each of the ten digits used to identify the item is represented by a specific group of these bars and the actual encoding of the digit is obtained by variation in widths of bars making up this group.
In some cases, lesser numbers of digits are used and provisions have been made for utilizing greater numbers of digits for future codification.
The reader-scan system contributes to the efficiency and convenience of the operation of automated check-out counters by allowing the UPC symbols to be read automatically as a package is manually transferred from the counter, across a scan pattern area or window.
In automatic electronic cash register systems, the data covering such things as pricing, quantity or coupon discounting and taxable or non-taxable nature of the item are stored in a memory bank of a controller console. The controller is programmed so that the address of this memory bank location corresponds to digital information encoded in the UPC symbol printed on the package of the item.
Typically, the scan pattern system uses a very low-powered laser, such as a helium-neon laser, to provide a coherent beam of monochromatic light. This type of light source provides the high level signal-to-noise ratio necessary for processing that is unavailable from other sources. The laser beam is then directed to a scanner mechanism which generates an optical scan pattern at a window in the check-out counter.
The actual identification of the symbol is made by electronically analyzing the signals generated by the laser light beam that is reflected back from the package surface to an optical detector which, in prior art systems, is typically a photo-multiplier tube. The output of this tube then goes to electronic circuitry and is continuously analyzed for the UPC symbol coded content.
When the high speed movement of the light beam crosses the light and dark bars of a UPC symbol, a specific pulse train waveform is generated. The characteristics of this waveform are established by the width of the individual light and dark bars and by the speed of the sweep. If the electronic circuitry determines that the symbol is valid and positive identification of the symbol is made, the signal is passed on to the controller of the cash register system. This output signal provides the address for the memory bank location where the instructions, for billing and cash register-receipt recording of that symbol, are stored.
If the symbol is not valid, i.e. has been tampered with, the positive identification cannot be made, a no-reading visual or audio alarm is sounded. This notifies the clerk that a visual identification and a manual cash register entry must be made.
The information from the UPC code is retrieved by detecting the diffusely scattered light, which is then spatially modulated in intensity as a function of the absorption coefficient of the inks used on the label. There is also a component of fresnel or specular reflection; this contains no information because, while inks have widely varying absorption coefficients, variations in reflectivity are slight, except with metallic materials. Optical reading of graphic symbols, therefore, depends on illumination of the label, detection of diffuse scattering, and demodulation of the spatially-intensity distribution as a function of absorption.
Reading the symbol is further complicated by the variety of ways in which the package can be moved across the scanning aperture in the check-out counter. This variety requires that the scanner has sufficient depth of field to read labels at varying distances from the reading aperture, sufficient speed to read at symbol velocities up to 100 inches per second, and pattern density sufficient to allow reading at random orientations and positions across the aperture. The reader must also have high resolution capable of reading graphically printed symbols having several line-pairs per millimeter.
Of importance is the optical scan pattern projected on the counter window or aperture. The laser beam spot must be small enough to resolve the line width of the UPC symbols. The scan pattern must have at least several inches depth of field, insuring that the UPC symbol will be scanned by the pattern even as it is moving at a rate of up to one hundred inches per second, the ability to read symbols on the side or bottom of a package, the ability to read symbols on a package rotating while moved across the scanner and insensitivity to ambient light so that the working area can be sufficiently bright as required in the store. At the same time, the laser power must be low enough to meet federal and state safety standards.
Additionally, the system must be designed so as to minimize the deleterious effect of specular reflection, to provide sufficient capture of reflected light from the scanned object, and utilize a minimum amount of moving parts to minimize expense and insure reliability.
Presently, there are at least three scan patterns utilized in optical reader-scanner systems. These are shown and described in an article entitled "Reading the Supermarket Code" by A. Hildebrand, Laser Focus, September 1974, pp. 10-18. These three patterns are referred to as the x, the sinusoidal or lissajous pattern, and the switch pattern.
The x scan pattern is probably the easiest to generate of all of these scans. On the negative side, this pattern requires a large window depth, may "lose" labels if they are rotated during passage and also achieves only a few scans of labels in many orientations.
The sinusoidal or lissajous pattern is generated by two reciprocating mirrors, or one reciprocating and one spinning mirror, and is a natural outgrowth of the desire for a narrower window. The resulting pattern is essentially a series of overlapping smaller crosses. Such a system is described in an article "Laser Scan Identifies Supermarket Items" by E. J. Stefanides, Design Ideas, Feb. 3, 1975. pp. 28-9.
This pattern is moderate in its difficulty of generation. It is reasonably efficient in the number of scans projected, but suffers near the peaks of the sine wave due to speed variation. It achieves only a few scans of labels in certain orientations.
The so-called "stitch" pattern consists of a series of parallel vertical line segments and a horizontal line segment, with respect to the direction of travel of the package across the counter window. This pattern also has significant deficiencies. It does not achieve a uniformly high number of scans for all orientations of a package passing over the scan pattern window. It requires orientation for magnified labels as well as truncated labels. It will not read labels with imperfections along a given line if the label is moving along that line. It provides little immunity to window damage or specular reflection. The system has low immunity to background light.
The optical systems for generating each of the scan patterns referred to above have several other things in common which detract from their performance. In each case, the scan lines forming the pattern all originate from the same point in space. In other words, if one were to view the pattern as it passes over the scan pattern window, it would appear that the position of the source of the light is the same for each of the segments of the scan pattern. This has several important disadvantages.
First, it means that the output power of the laser must be reduced in order to meet safety power requirements. Secondly, since the light originates from the same point, the scans on the scan window all strike the window at approximately the same angle. In other words, all of the lines forming the pattern come from approximately the same direction. This can result in problems caused by specular reflection. Having the light beam originate from the same point also makes it less efficient in trying to scan labels which are poorly oriented as they pass the scan window. This is particularly true of curved labels.
Another common characteristic of the delivery systems for producing these patterns is that the optical system for gathering light reflected from the labels is independent of the optics delivering the laser beam to the label. By this, it is meant that the reflected light does not follow an optical path common with the impinging light. Thus, while the laser beam impinges upon the target in the form of a narrow beam of light, the optical detection scheme is provided which must have a large collection-cone angle to enhance detectivity of the reflected light. Unfortunately, this also means that additional ambient light is introduced in addition to the light providing the label information. This makes filters necessary to cut out the background light. Since this approach requires the imaging a large volume in space, very sensitive light detectors, namely photo multipliers, are required. This introduces a significant expense into the detection apparatus.